1. Field of the Invention
The invention relates to a liquid crystal display, and more particularly to a liquid crystal display which is capable of preventing display defect caused by steps formed on inner surfaces of substrates. The invention relates also to a method of preventing display defect caused by alignment defect in a liquid crystal display.
2. Description of the Related Art
There has been widely used a twist nematic (TN) type liquid crystal display.
FIG. 1 illustrates a structure of a conventional liquid crystal display. The illustrated liquid crystal display is comprised of a lower substrate 5, an upper substrate (not illustrated) joined to the lower substrate 5 with a certain space being kept therebetween, and a liquid crystal layer 1 sandwiched between the lower substrate 5 and the upper substrate.
As illustrated in FIG. 1, an interlayer insulating film 4, a pixel electrode film 3, and an insulating film 2 are deposited on the lower substrate 5 in this order.
There are formed a lot of steps on inner surfaces of the lower and upper substrates with which the liquid crystal layer 1 makes contact. In particular, steps of the insulating film 2 formed on an inner surface of the lower substrate 5 cause reverse tilt and reverse twist which often extend to a display area of a liquid crystal display to thereby cause display defect with the result of deterioration in quality of images.
Since alignment treatment such as rubbing is conducted along steps formed on inner surfaces of the substrates, the liquid crystal layer 1 is put in reverse tilt condition in an inclined portion B which is shaded in a direction A in which alignment treatment is conducted. As a result, there occurs such display defect as mentioned above.
A lot of solutions to such a problem have been suggested so far.
The first solution is to form a film on inner surfaces of substrates by overcoating to thereby eliminate steps or flatten inner surfaces.
However, this solution is accompanied with a problem of deterioration in lead time caused by additional fabrication steps, and a problem of an increase in fabrication cost.
The second solution is to increase a pre-tilt angle .theta.).
However, it is generally difficult to make a pre-tilt angle .theta. greater than an angle .phi. of steps. Hence, it is quite difficult or almost impossible to prevent occurrence of the area B in which alignment is defective.
In addition, as a liquid crystal display has been fabricated smaller in size, an interval M between adjacent steps has become smaller. Hence, it is no longer possible to prevent occurrence of the area B only by making a pre-tilt angle .theta. greater.
Japanese Unexamined Patent Publication No. 5-265046 has suggested a liquid crystal display in which a side by which a pre-tilt angle of liquid crystal molecules can be obtained, among adjacent two sides of a pixel electrode, and the other side by which a pre-tilt angle in a reverse direction can be obtained are arranged in certain positions to thereby prevent alignment defect in liquid crystal.
Japanese Unexamined Patent Publication No. 7-64096 has suggested a liquid crystal display in which a pre-tilt direction is controlled so that a direction in which liquid crystal molecules are oriented by electric field running in a thickness-wise direction of a liquid crystal layer is common to a direction in which liquid crystal molecules are oriented by electric field, to thereby prevent deterioration in display quality.
However, the liquid crystal displays suggested in the above-mentioned Publications attempt to prevent alignment defect in liquid crystal by controlling a pre-tilt angle as a sole parameter. Hence, for the same reason as mentioned earlier, the suggested liquid crystal displays could not prevent occurrence of the area B.